The mammalian heart is well supplied by autonomic nerves which harmonize the cardiac rhythmic activities. Recently, at least 7 neuropeptides have been found to occur in the nerves of the heart. This raised a number of important questions that await answers. How many types of nerves as distinguished by their neurotransmitters/neuromodulators are in the heart? What is the population, distribution and origin of each nerve type? What is the extent of the co-storage of two or more different neurotransmitters/neuromodulators in those nerves? Where are those neuronal substances synthesized? The occurrence, abundance and distributions of neuropeptide Y, calcitonin gene-related peptide, substance P, vasoactive intestinal polypeptide, enkephalin, neurotensin, and somatostatin in the heart will be investigated comparatively using light microscopic immunocytochemistry on consecutive sections. In addition, the abundance and distribution of dopamine-hydroxylase, tyrosine hydroxylase and acetylcholine, neuron specific-enolase and S-100 will be simultaneously studies for the purpose of comparison. The ultrastructural location and possible co-storage of the above substances in intrinsic neurons will be examined by RNA-RNA tissue in situ hybridization. The results from this study will provide a detailed morphologic basis for a more complete understanding of the functional significance of neuropeptides in the cardiac innervation.